Sudden death has been reported in patients with multiple system atrophy (MSA), although the frequency of this event has not been well delineated. We investigated the frequency and potential causes of sudden death in patients with MSA. During the 5-year observation period, 10 of 45 patients with probable MSA died. The causes of death included sudden death of unknown etiology (seven patients), aspiration pneumonia (one patient), asphyxia after vomiting (one patient), and lung cancer (one patient). The mean survival time of patients with sudden death was 63.0 +/- 24.7 months (range, 39-116 months). Among seven patients who experienced sudden death, six were found to have died during sleep. Among these patients, two had been treated with tracheostomy and three with continuous positive airway pressure (CPAP) or noninvasive positive pressure ventilation (NPPV) during sleep, suggesting that these treatments do not always prevent sudden death in patients with MSA. Nocturnal sudden death should be recognized as the most common mechanism of death in patients with MSA.